1. Field of the Invention
The present invention relates to fibrous magnesium oxysulfate of granular form and a process for producing the magnesium oxysulfate, as well as to a thermoplastic resin composition containing magnesium oxysulfate.
It is a well known technique to add an inorganic substance to a polypropylene resin in order to improve the latter's properties such as rigidity, mechanical strengths, heat resistance, mold shrinkage factor, dimensional stability and the like. As such an inorganic substance, particulate substances (e.g. calcium carbonate, barium sulfate, magnesium hydroxide), small plate-shaped or flaky substances (e.g. talc, mica) and fibrous substances (e.g. glass fiber, asbestos) are well known.
The particulate substances show no sufficient reinforcing effect for polypropylene resins. Therefore, the polypropylene resin compositions obtained by adding such a particulate substance to a polypropylene resin cannot be used ordinarily in applications where a high performance is required. Meanwhile, the small plate-shaped or flaky substances and the fibrous substances show an excellent two or one-dimensional reinforcing effect for polypropylene resins and accordingly are in wide use as a reinforcing material for polypropylene resins. These materials showing a high reinforcing effect, however, have drawbacks in other respects, thus restricting their use as a reinforcing material depending upon their applications. For example, the polypropylene resins containing a small plate-shaped or flaky substance, as compared with those containing a fibrous substance (e.g. glass fiber), generally has low rigidity when made into a molded article and easily give rise to flow marks during molding. The polypropylene resins containing a fibrous substance, when made into a molded article, show small elongation, easily cause silver streaking, and give poor luster. The polypropylene resin containing a glass fiber, in particular, shows low impact resistance at high temperatures when made into a molded article. Hence, in adding these conventional reinforcing materials to polypropylene resins, the selection of a reinforcing material must be made in consideration of not only the application of the molded article obtained from the resulting polypropylene resin composition but also the drawback of the reinforcing material.
The above mentioned drawbacks of the conventional reinforcing materials can be partly reduced by, for example, the selection and modification of a polypropylene resin used as a base material, the surface treatment of an inorganic PG,5 substance used as a reinforcing material, the addition of an appropriate third substance and the modification of molding conditions. However, these measures cannot sufficiently reduce the above drawbacks of the conventional reinforcing materials.
Japanese Patent Application Kokai (Laid-Open) No. 57-109846 discloses that the use of fibrous magnesium oxysulfate as an inorganic substance can reduce the above drawbacks.
However, since the fibrous magnesium oxysulfate has a very small bulk specific gravity, its mixing with a pellet-like or granular resin is not easy (a polypropylene resin of pellet or granular form is generally used in ordinary compounding). That is, since the fibrous magnesium oxysulfate and the polypropylene resin have different shapes and densities, they are, as in the ordinary mixing of an inorganic substance and a polypropylene resin, premixed in a dry blender such as V-shaped blender, ribbon mixer or the like and then fed to a melt mixer (this is a dry blending method); in this case, there easily occurs, at the inlet of the melt mixer, bridging or the separation of the polypropylene resin and the fibrous magnesium oxysulfate. This reduces the productivity of a desired reinforced resin composition and makes it difficult to obtain a uniform resin composition; as a result, the molded articles produced therefrom have poor properties and poor appearance in some cases.
Japanese Patent Application Kokai (Laid-Open) No. 59-172533 discloses a process for producing a reinforced resin composition free from the above mentioned drawbacks, by mixing a thermoplastic resin and fibrous magnesium oxysulfate.
This process comprises feeding into a melt kneader a pellet-like or granular thermoplastic resin and fibrous magnesium oxysulfate separately through respective inlets and then melt-kneading them in the kneader.
Also in this process, however, since the fibrous magnesium oxysulfate has a very small bulk specific gravity, the uniform feeding of the magnesium oxysulfate into the kneader is difficult; as a result, it is difficult to obtain a resin composition of good properties and good appearance.